Turbocharger with stepped two-stage vane nozzle

ABSTRACT

A turbocharger having a variable nozzle with stepped two-stage vanes ( 50 ), the variable nozzle comprising a tubular piston ( 70 ) disposed in the bore ( 44 ) of the turbine housing ( 38 ) such that the piston ( 70 ) is axially slidable adjacent to the vanes ( 50 ) that extend across the nozzle. Each vane defines a first vane stage ( 50   a ) proximate the free end of the vane, the second vane stage ( 50   b ) having a different aerodynamic contour in comparison with the first vane stage ( 50   a ), each vane comprising a step ( 60 ) transitioning from the first vane stage ( 50   a ) to the second vane stage ( 50   b ). The piston ( 70 ) in a closed position closes the second vane stage ( 50   b ) so that exhaust gas flows only through the first vane stage ( 50   a ). The second vane stage ( 50   b ) is progressively opened as the piston ( 70 ) is axially slid toward an open position.

BACKGROUND OF THE INVENTION

The present invention relates generally to turbochargers, and relatesmore particularly to exhaust gas-driven turbochargers having an axiallysliding piston for varying the size of a nozzle that leads into theturbine wheel so as to regulate flow into the turbine wheel.

Regulation of the exhaust gas flow through the turbine of an exhaustgas-driven turbocharger provides known operational advantages in termsof improved ability to control the amount of boost delivered by theturbocharger to the associated internal combustion engine. Theregulation of exhaust gas flow is accomplished by incorporating variablegeometry into the nozzle that leads into the turbine wheel. By varyingthe size of the nozzle flow area, the flow into the turbine wheel can beregulated, thereby regulating the overall boost provided by theturbocharger's compressor.

Variable-geometry nozzles for turbochargers generally fall into two maincategories: variable-vane nozzles, and sliding-piston nozzles. Vanes areoften included in the turbine nozzle for directing the exhaust gas intothe turbine in an advantageous direction. Typically a row ofcircumferentially spaced vanes extend axially across the nozzle. Exhaustgas from a chamber surrounding the turbine wheel flows generallyradially inwardly through passages between the vanes, and the vanes turnthe flow to direct the flow in a desired direction into the turbinewheel. In a variable-vane nozzle, the vanes are rotatable about theiraxes to vary the angle at which the vanes are set, thereby varying theflow area of the passages between the vanes.

In the sliding-piston type of nozzle, the nozzle may also include vanes,but the vanes are fixed in position. Variation of the nozzle flow areais accomplished by an axially sliding piston that slides in a bore inthe turbine housing. The piston is tubular and is located just radiallyinwardly of the nozzle. Axial movement of the piston is effective tovary the axial extent of the nozzle opening leading into the turbinewheel. When vanes are included in the nozzle, the piston can slideadjacent to radially inner (i.e., trailing) edges of the vanes;alternatively, the piston and vanes can overlap in the radial directionand the piston can include slots for receiving at least a portion of thevanes as the piston is slid axially to adjust the nozzle opening.

One of the design challenges with such sliding piston-type nozzles is tooptimize the aerodynamics of the exhaust gas flow into the turbine wheelover the full stroke of the piston. In some sliding piston-type variablenozzles, flow disturbance can occur particularly in the beginning ofpiston stroke as the piston begins to move from its closed positiontoward a more-open position. More particularly, as the piston begins toopen even a very small amount, the flow rate into the turbine cansuddenly increase, making it difficult to regulate the piston strokewith sufficient accuracy to prevent a sudden flow surge.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses the above needs and achieves otheradvantages, by providing a turbocharger having a sliding piston-typevariable nozzle in which stepped two-stage vanes are employed. Inaccordance with one embodiment of the invention, the turbochargerincludes a turbine wheel disposed in a bore of a turbine housing, theturbine housing defining a chamber surrounding the turbine wheel forreceiving exhaust gas to be directed into the turbine wheel, a radiallyinner side of the chamber having an axial length. The turbochargerfurther comprising a variable nozzle having stepped two-stage vanes, thevariable nozzle comprising a tubular piston disposed in the bore of theturbine housing such that the piston is axially slidable relative to theturbine housing along the radially inner side of the chamber such thatthe piston blocks a variable portion of the axial length of the chamberdepending on axial position of the piston, the piston having an upstreamend and a downstream end with respect to a flow direction of exhaust gasalong the bore of the turbine housing. A generally annular wall extendsgenerally radially inwardly adjacent an upstream end of the axial lengthof the chamber, and an array of circumferentially spaced vanes havefixed ends joined to the generally annular wall and opposite free ends,the vanes extending across the axial length of the chamber. Each of thevanes has an outer surface that faces generally radially outwardly andan opposite inner surface that faces generally radially inwardly. Eachvane defines a first vane stage proximate the fixed end of the vane anda second vane stage proximate the free end of the vane, the second vanestage having a different aerodynamic contour of at least one of theouter and inner surfaces in comparison with the first vane stage, eachvane comprising a step transitioning from the first vane stage to thesecond vane stage. The piston in a closed position closes the secondvane stage so that exhaust gas flows only through the first vane stage.The second vane stage is progressively opened as the piston is axiallyslid toward an open position.

In accordance with the invention, the first vane stage can beaerodynamically tailored to optimize turbocharger performance at lowengine speeds where the exhaust gas flow rate is relatively low. Thesecond vane stage can be designed to optimize turbocharger performanceat higher engine speeds where exhaust gas flow rates are substantiallygreater. The stepped two-stage vane design thus provides a greaterdegree of design flexibility.

In one embodiment of the invention, the step in each vane defines adownstream-facing surface that is abutted by the piston to define theclosed position of the piston. The step can be defined in the outersurface of each vane, and the upstream end of the piston can have aradially inner surface that travels adjacent to the outer surfaces ofthe second vane stages as the piston is axially slid. Thus, the secondvane stages are received into the central passage of the tubular pistonas the piston moves toward the closed position.

Alternatively, the step can be defined in the inner surface of eachvane, and the upstream end of the piston can have a radially outersurface that travels adjacent to the inner surfaces of the second vanestages as the piston is axially slid. In this variation, the piston issmaller in diameter than the vane array and the second vane stagesreside adjacent the radially outer surface of the piston as the pistonmoves toward the closed position.

In another embodiment, the step is in the outer surface of each vane,and the upstream end of the piston has a radially outer surface in whichrecesses are formed for respectively receiving the second vane stageswith the inner surface of each second vane stage confronting a radiallyoutwardly facing wall of each respective recess. The free ends of thevanes abut end walls of the recesses to define the closed position ofthe piston. The engagement of the second vane stages in the recesses ofthe piston serves to prevent rotation of the piston.

According to a further embodiment, the upstream end of the piston has aradial wall thickness exceeding a radial extent of the second vanestages, and the piston defines slots extending into the upstream end ofthe piston for receiving the second vane stages. The step in the vanescan abut the end of the piston to define the closed position of thepiston. The step can be in the outer surface of each vane, oralternatively the step can be in the inner surface or in both the outerand inner surfaces.

In yet another embodiment of the invention, the upstream end of thepiston defines a radially outwardly extending flange, the flangedefining apertures therethrough for receiving the second vane stages.The flange can abut the step in the vanes to define the closed positionof the piston. The step can be defined in both outer and inner surfacesof the vanes, or alternatively can be in one or the other of thesurfaces.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a cross-sectional view of a turbine assembly for aturbocharger, in accordance with a first embodiment of the invention,showing the piston in a closed position;

FIG. 2 is a cross-sectional view along line 2-2 in FIG. 1;

FIG. 3 is a view similar to FIG. 1, showing the piston in an openposition;

FIG. 4 is a cross-sectional view along line 4-4 in FIG. 3;

FIG. 5 is a cross-sectional view of a turbine assembly for aturbocharger, in accordance with a second embodiment of the invention,showing the piston in a closed position;

FIG. 6 is a cross-sectional view along line 6-6 in FIG. 5;

FIG. 7 is a view similar to FIG. 5, showing the piston in an openposition;

FIG. 8 is a cross-sectional view along line 8-8 in FIG. 7;

FIG. 9 is a cross-sectional view of a turbine assembly for aturbocharger, in accordance with a third embodiment of the invention,showing the piston in a closed position;

FIG. 10 is a cross-sectional view along line 10-10 in FIG. 9;

FIG. 11 is a view similar to FIG. 9, showing the piston in an openposition;

FIG. 12 is a cross-sectional view along line 12-12 in FIG. 11;

FIG. 13 is a cross-sectional view of a turbine assembly for aturbocharger, in accordance with a fourth embodiment of the invention,showing the piston in a closed position;

FIG. 14 is a cross-sectional view along line 14-14 of FIG. 13;

FIG. 15 is a cross-sectional view of a turbine assembly for aturbocharger, in accordance with a fifth embodiment of the invention,showing the piston in a closed position;

FIG. 16 is a cross-sectional view along line 16-16 of FIG. 15;

FIG. 17 is a cross-sectional view of a turbine assembly for aturbocharger, in accordance with a sixth embodiment of the invention,showing the piston in a closed position; and

FIG. 18 is a cross-sectional view along line 18-18 of FIG. 17.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings in which some but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

FIGS. 1 through 4 illustrate a turbine assembly 20 for a turbocharger,having a sliding piston-type variable nozzle in accordance with a firstembodiment of the invention. The turbine assembly includes a turbinehousing 38 adapted to be coupled to a center housing (not shown) of theturbocharger. A turbine wheel (not shown) is mounted on an end of arotatable shaft (not shown) of the turbocharger and is disposed in theturbine housing. The turbine housing defines a chamber 42 that surroundsthe turbine wheel and receives exhaust gas from the internal combustionengine. Exhaust gas is directed from the chamber 42 into the turbinewheel, which expands the exhaust gas and is driven thereby so as todrive a compressor wheel (not shown) mounted on the opposite end of theshaft. The chamber 42 at its radially inner side has an axial extent,defined between opposite walls 41, 43 of the chamber, that generallycorresponds to an axial extent of the blades of the turbine wheel,although in some cases it may be desirable for the axial extent of thechamber to be somewhat greater than that of the turbine blades to allowfor the possibility of some proportion of exhaust gas flow to bypass theturbine blades, as further described below. The opening defined betweenthe walls 41, 43 of the chamber makes up part of what is referred toherein as the turbine nozzle.

The turbine housing 38 defines a bore 44 whose diameter generallycorresponds to a radially innermost extent of the chamber 42. Theturbine wheel resides in an upstream end of the bore 44 and the turbinewheel's rotational axis is substantially coaxial with the bore. The term“upstream” in this context refers to the direction of exhaust gas flowthrough the bore 44, as the exhaust gas in the chamber 42 flows into theturbine wheel and is then turned to flow generally axially (left toright in FIG. 1) through the bore 44 to its downstream end. Thus,“upstream” is to the left and “downstream” is to the right in FIG. 1.

The turbocharger 20 includes a heat shield 46 that is mounted betweenthe turbine housing 38 and the center housing and provides a barriersubstantially preventing hot exhaust gas from infiltrating orexcessively heating up the bearing area in the center housing. The heatshield comprises a generally annular wall 48 that extends generallyradially inwardly from, and essentially serves as an extension of, thewall 41 of the turbine housing chamber 42, adjacent an upstream end ofthe turbine wheel. Axially projecting from the wall 48 is an array ofcircumferentially spaced vanes 50. The vanes have fixed ends joined tothe wall 48, and opposite free ends. The length of the vanes from thewall 48 to the free ends of the vanes substantially corresponds to theaxial length defined between the walls 41, 43 of the turbine housingchamber at its radially inner side. Thus, the vanes 50 extendsubstantially all the way across the opening between the walls 41, 43,and the free ends of the vanes can abut the wall 43 if desired.Alternatively, however, the vanes could be shorter than the axial lengthof the chamber opening.

Each vane 50 comprises a stepped vane. More specifically, withparticular reference to FIGS. 3 and 4, each vane has an outer surface 52that faces generally radially outwardly, and an inner surface 54 thatfaces generally radially inwardly. Each vane has a leading edge 56 and atrailing edge 58 with respect to a direction of flow of exhaust gasthrough the spaces defined between adjacent vanes. The outer surface 52of each vane has a step 60 that extends from the leading edge 56 partwaytoward the trailing edge 58. The step 60 has its maximum height at theleading edge 56 and gradually diminishes in height with distance alongthe vane toward the trailing edge 58 until the step vanishes. The step60 divides the vane into two portions: a first vane stage 50 a thatextends from the fixed end of the vane at the wall 48 up to the step 60;and a second vane stage 50 b that extends from the step 60 to the freeend of the vane. The step 60 creates an axially facing surface 62 thatfaces generally toward the turbine chamber wall 43. As further describedbelow, the surfaces 62 of the stepped vanes serve as a stop for anaxially sliding piston of the variable nozzle mechanism.

The turbine assembly 20 further comprises an axially sliding piston 70of tubular form. The piston is disposed within the bore 44 of theturbine housing and is axially slidable in the bore. At least onesealing ring is disposed in a groove in the radially outer surface ofthe piston for engaging the inner surface of the bore 44 to providesealing between the piston and bore so that exhaust gas cannot escapebetween these parts. The piston is slidable between a closed position(FIG. 1) and an open position (FIG. 3), and various intermediatepositions therebetween. The inner diameter of at least the upstream endportion of the piston that axially overlaps the second vane stage 50 bis slightly greater than the maximum diameter of the second vane stage.The piston's radially inner surface travels adjacent to the outersurfaces 52 of the second vane stages 50 b as the piston is axiallyslid. In the closed position of the piston, an upstream end surface 74of the piston abuts the stop surfaces 62 of the stepped vanes. In thisposition, the piston effectively blocks the entire second vane stage 50b so that exhaust gas is prevented from flowing through the second vanestage to the turbine wheel. Accordingly, apart from a very small amountof leakage of exhaust gas that may occur between the stop surfaces 62and the piston end surface 74 and then through the second vane stages 50b, substantially all of the exhaust gas that enters the turbine wheelflows through the first vane stage 50 a when the piston is closed.

In the open position (FIG. 3) of the piston 70, the upstream end surface74 can be generally in axial alignment with the wall 43 of the turbinehousing chamber, or can be somewhat upstream of or downstream of thewall 43. When the piston is axially slid in the downstream directionstarting from the closed position, the end surface 74 of the pistonseparates from the stop surfaces 62 of the stepped vanes, andaccordingly exhaust gas begins to flow through the unblocked portion ofthe second vane stage 50 b. With further travel of the piston, more andmore of the second vane stage is unblocked so that a greater and greaterproportion of the total exhaust gas flow passes through the second vanestage. When the piston reaches the open (i.e., fully open) position,preferably the entire length of the second vane stage is unblocked.

In accordance with the invention, the first vane stage 50 a has adifferent aerodynamic contour than the second vane stage 50 b. The firstvane stage can be optimized for low engine speeds where exhaust gas flowrates are relatively low. The second vane stage can be optimized forhigher engine speeds where exhaust gas flow rates are higher.

The presence of the second vane stage 50 b, in comparison with having novanes at all in the second nozzle portion, allows a smoother turbineflow rate evolution as the piston just begins to open from its fullyclosed position. With no vanes present, the turbine flow rate tends tosuddenly increase when the piston just begins to open. This makes itdifficult to regulate the piston stroke with sufficient accuracy toprevent a sudden flow surge. The result can be poor vehicle behavior asthe degree of turbocharging provided to the engine suddenly increases.The presence of the second vane stage 50 b helps prevent such suddenflow surge. Additionally, as noted, the second vane stage can have adifferent aerodynamic contour compared to the first vane stage. Thisallows an optimization of the vane incidence and the flow angle exitingthe vanes into the turbine stage for each of the first and second vanestages, thereby enhancing design flexibility.

FIGS. 5 through 8 illustrate a turbine assembly 20′ of a turbocharger inaccordance with a second embodiment of the invention. The turbineassembly 20′ is generally similar to the turbine assembly 20 describedabove. The turbine assembly 20′ includes a turbine housing 38 defining achamber 42 and having a bore 44 generally as in the prior embodiment. Aheat shield 46 is mounted between the turbine housing 38 and the centerhousing (not shown) and defines a generally annular wall 48 as in theprior embodiment. An array of circumferentially spaced stepped two-stagevanes 50′ are joined to the wall 48 and project axially therefrom acrossthe radially inner side of the chamber 42, with free ends of the vanesclosely adjacent or abutting the wall 43 of the chamber.

Each vane has an outer surface 52′ that faces generally radiallyoutwardly, and an inner surface 54′ that faces generally radiallyinwardly. Each vane has a leading edge 56′ and a trailing edge 58′ withrespect to a direction of flow of exhaust gas through the spaces definedbetween adjacent vanes. The inner surface 54′ of each vane has a step60′ that extends from the trailing edge 58′ partway toward the leadingedge 56′. The step 60′ has its maximum height at the trailing edge 58′and gradually diminishes in height with distance along the vane towardthe leading edge 56′ until the step vanishes. The step 60′ divides thevane into two portions: a first vane stage 50 a′ that extends from thefixed end of the vane at the wall 48 up to the step 60′; and a secondvane stage 50 b′ that extends from the step 60′ to the free end of thevane. The step 60′ creates an axially facing surface 62′ that facesgenerally toward the turbine chamber wall 43. As further describedbelow, the surfaces 62′ of the stepped vanes serve as a stop for anaxially sliding piston of the variable nozzle mechanism.

The variable nozzle mechanism further comprises an axially slidingpiston 70′ of tubular form. The mechanism also includes a carrier 80 oftubular form that is axially inserted into the bore 44 of the turbinehousing and retained in a fixed position therein by a suitable retainingmechanism such as a snap ring 82 or the like. The carrier 80 defines acentral bore within which the piston is received such that the piston isaxially slidable relative to the carrier. At least one sealing ring isdisposed in a groove in the radially outer surface of the piston forengaging the inner surface of the carrier 80 to provide sealing betweenthe piston and carrier so that exhaust gas cannot escape between theseparts. The carrier advantageously can be split along an axial line sothat the carrier can radially expand and contract along with the pistonso as to prevent binding or excessive clearance therebetween. The pistonis slidable between a closed position and an open position, and variousintermediate positions therebetween. The outer diameter of at least theupstream end portion of the piston that axially overlaps the second vanestage 50 b′ is slightly smaller than the minimum diameter of the secondvane stage. The piston's radially outer surface travels adjacent to theinner surfaces 54′ of the second vane stages 50 b′ as the piston isaxially slid In the closed position of the piston, an upstream endsurface 74′ of the piston abuts the stop surfaces 62′ of the steppedvanes. In this position, the piston effectively blocks the entire secondvane stage 50 b′ so that exhaust gas is prevented from flowing throughthe second vane stage to the turbine wheel. Accordingly, except for asmall amount of leakage flow that may escape between the stop surfaces62′ and the piston end surface 74′ and then flow through the second vanestage 50 b′, exhaust gas can flow only through the first vane stage 50a′ when the piston is closed.

In the open position of the piston 70′, the upstream end surface 74′ canbe generally in axial alignment with the wall 43 of the turbine housingchamber, or can be somewhat upstream of or downstream of the wall 43.When the piston is axially slid in the downstream direction startingfrom the closed position, the end surface 74′ of the piston separatesfrom the stop surfaces 62′ of the stepped vanes, and accordingly exhaustgas begins to flow through the unblocked portion of the second vanestage 50 b′. With further travel of the piston, more and more of thesecond vane stage is unblocked so that a greater and greater proportionof the total exhaust gas flow passes through the second vane stage. Whenthe piston reaches the open (i.e., fully open) position (FIG. 7),preferably most or all of the length of the second vane stage isunblocked.

FIGS. 9 through 12 show a turbine assembly 120 of a turbocharger inaccordance with a third embodiment of the invention. The turbineassembly is generally similar to those of the first and secondembodiments described above. The turbine assembly includes a turbinehousing 38 having a chamber 42 and defining a bore 44, and includes aheat shield 46 having vanes 50 that include a step 60 in the vane outersurfaces defining stop surfaces 62 as in the first embodiment. Thevariable nozzle mechanism includes a carrier 80 as in the secondembodiment above. The primary differences of the third embodimentcompared to the first and second embodiments reside in the piston 170that slides within the carrier 80. The piston 170 at its upstream enddefines recesses 176 in the radially outer surface of the piston forreceiving the second vane stages 50 b of the vanes. The recesses extendonly partially through the radial thickness of the piston wall, suchthat each recess has a bottom surface that faces the inner surface 54 ofthe respective vane received in the recess. The engagement of the vanesin the recesses tends to prevent the piston from rotating about itsaxis, and also helps to improve the sealing between the piston and thesecond vane stages so that exhaust gas is deterred from leaking betweenthese parts. The recesses advantageously have an axial lengthcorresponding to that of the second vane stage 50 b and terminate at anend wall 178 of each recess. The free ends of the vanes abut the recessend walls 178 when the piston is in its closed position; in thisposition, the end surface 174 of the piston is axially aligned with thesteps 60 in the vanes, and hence the piston blocks the entire secondvane stage 50 b such that exhaust gas flows only through the first vanestage 50 a to the turbine wheel. As the piston is slid axiallydownstream, a gap begins to open up between the end surface of thepiston and the steps 60 so that exhaust gas begins to flow through theunblocked portion of the second vane stage. When the piston reaches theopen position, preferably the entire length of the second vane stage isunblocked.

FIGS. 13 and 14 illustrate a turbine assembly 220 of a turbocharger inaccordance with a fourth embodiment of the invention. The turbineportion is generally similar to that of the third described above. Theturbine portion includes a turbine housing 38 having a chamber 42 anddefining a bore 44, and includes a heat shield 46 having vanes 50 thatinclude a step 60 in the vane outer surfaces defining stop surfaces 62just as in the third embodiment. The primary differences of the fourthembodiment compared to the third embodiment reside in the piston 270that slides within the turbine housing bore 44. The piston 270 at itsupstream end has a greater radial wall thickness than in the thirdembodiment, and defines slots 276 that extend axially into the pistonfrom the upstream end surface 274 for receiving the second vane stages50 b of the vanes. The engagement of the vanes in the slots tends toprevent the piston from rotating about its axis and helps providesealing between the piston and vanes. The slots advantageously have anaxial length at least as great as, and preferably substantially equalto, that of the second vane stage 50 b. The end surface 274 of thepiston abuts the vane stop surfaces 62 when the piston is in its closedposition; in this position, the piston blocks the entire second vanestage 50 b such that exhaust gas flows only through the first vane stage50 a to the turbine wheel. As the piston is slid axially downstream, agap begins to open up between the end surface of the piston and the stopsurfaces 62 so that exhaust gas begins to flow through the unblockedportion of the second vane stage. When the piston reaches the openposition, preferably the entire length of the second vane stage isunblocked.

FIGS. 15 and 16 depict a turbine assembly 320 of a turbocharger inaccordance with a fifth embodiment of the invention. This embodiment isgenerally similar to the third embodiment described above. The turbineassembly includes a turbine housing 38 having a chamber 42 and defininga bore 44. The turbine portion also includes a heat shield 346 havingvanes 350, but unlike the third embodiment, the vanes include a step inboth their outer surfaces and their inner surfaces, defining stopsurfaces and dividing the vanes into first vane stages 350 a and secondvane stages 350 b. The variable nozzle mechanism includes a carrier 80and the piston 370 slides within the carrier as in the second embodimentabove. The piston 370 at its upstream end has a radially outwardlyextending flange 378. The flange defines apertures 377 for receiving thesecond vane stages 350 b of the vanes. The radially outer surface of thepiston just downstream of the flange 378 also defines recesses 376 forreceiving the second vane stages, similar to the recesses 176 of thethird embodiment described above. The engagement of the vanes in theapertures 377 of the flange and in the recesses 376 tends to prevent thepiston from rotating about its axis and helps provide sealing betweenthe piston and vanes. The flange 378 abuts the vane stop surfaces whenthe piston is in its closed position as shown in FIG. 15; in thisposition, the end surface 174 of the piston flange is axially alignedwith the steps 360 in the vanes, and hence the piston blocks the entiresecond vane stage 350 b such that exhaust gas flows only through thefirst vane stage 350 a to the turbine wheel. As the piston is slidaxially downstream, a gap begins to open up between the end surface ofthe piston and the steps 360 so that exhaust gas begins to flow throughthe unblocked portion of the second vane stage. When the piston reachesthe open position, preferably the entire length of the second vane stageis unblocked.

FIGS. 17 and 18 show a sixth embodiment of the invention. The turbineassembly 420 in accordance with this embodiment has a heat shield 346supporting vanes 350 substantially identical to those of the fifthembodiment above. The turbine portion includes a piston 470 that slidesin the bore 44.of the turbine housing 38. The piston 470 issubstantially similar to the piston 270 of the fourth embodiment. Thus,the piston 470 at its upstream end has a substantial radial wallthickness and defines slots 476 that extend axially into the piston fromthe upstream end surface 474 for receiving the second vane stages 350 bof the vanes. The engagement of the vanes in the slots tends to preventthe piston from rotating about its axis and helps provide sealingbetween the piston and vanes. The slots advantageously have an axiallength at least as great as, and preferably substantially equal to, thatof the second vane stage 350 b. The end surface 474 of the piston abutsthe vane stop surfaces when the piston is in its closed position; inthis position, the piston blocks the entire second vane stage 350 b suchthat exhaust gas flows only through the first vane stage 350 a to theturbine wheel. As the piston is slid axially downstream, a gap begins toopen up between the end surface of the piston and the stop surfaces sothat exhaust gas begins to flow through the unblocked portion of thesecond vane stage. When the piston reaches the open (i.e., fully open)position, preferably most or all of the length of the second vane stageis unblocked.

In the various embodiments of the invention, in order to avoid bindingbetween the piston and the second stage vanes at high temperature, asmall gap is intentionally provided between the second stage vane outersurface(s) and corresponding any surface(s) of the piston confrontedthereby. As described, the first stage vanes have a “thicker” profilethan the second stage vanes, thus creating the so-called “step”. Thestep serves two functions: (1) it seals up the gap between the pistonand second stage vanes when the piston is fully closed, and (2) itensures a reliable mechanical stop for the sliding piston in the fullyclosed position.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. For example, thestepped vanes in the illustrated and described embodiments are affixedto a heat shield, but alternatively the turbocharger can be designedsuch that the vanes are affixed to a wall of the center housing or areaffixed in some other way. Therefore, it is to be understood that theinventions are not to be limited to the specific embodiments disclosedand that modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

1. A turbine assembly for a turbocharger, comprising: a turbine housingdefining a bore and defining a chamber surrounding the bore forreceiving exhaust gas to be directed into a turbine wheel disposed inthe bore, a radially inner side of the chamber having an axial length;and a variable nozzle having stepped two-stage vanes, the variablenozzle comprising: a tubular piston disposed in the bore of the turbinehousing such that the piston is axially slidable relative to the turbinehousing along the radially inner side of the chamber such that thepiston blocks a variable portion of the axial length of the chamberdepending on axial position of the piston, the piston having an upstreamend and a downstream end with respect to a flow direction of exhaust gasalong the bore of the turbine housing; and a generally annular wall thatextends generally radially inwardly adjacent an upstream end of theaxial length of the chamber, and an array of circumferentially spacedvanes having fixed ends joined to the generally annular wall andopposite free ends, the vanes extending across the axial length of thechamber, the vanes each having an outer surface that faces generallyradially outwardly and an opposite inner surface that faces generallyradially inwardly, wherein each vane defines a first vane stageproximate the fixed end of the vane and a second vane stage proximatethe free end of the vane, wherein the first vane stage is contoured toguide the exhaust gas to exit the first vane stage with a first flowangle, and the second vane stage is contoured to guide the exhaust gasto exit the second vane stage with a second flow angle different fromthe first flow angle, each vane comprising a step transitioning from thefirst vane stage to the second vane stage, and wherein the piston in aclosed position closes the second vane stage so that exhaust gas flowsonly through the first vane stage, the second vane stage beingprogressively opened as the piston is axially slid toward an openposition.
 2. The turbine assembly of claim 1, wherein the step defines adownstream-facing surface that is abutted by the piston to define theclosed position of the piston.
 3. The turbine assembly of claim 1,wherein the step is in the outer surface of each vane, and the upstreamend of the piston has a radially inner surface that travels adjacent tothe outer surfaces of the second vane stages as the piston is axiallyslid.
 4. The turbine assembly of claim 1, wherein the step is in theinner surface of each vane, and the upstream end of the piston has aradially outer surface that travels adjacent to the inner surfaces ofthe second vane stages as the piston is axially slid.
 5. The turbineassembly of claim 1, wherein the upstream end of the piston has a radialwall thickness exceeding a radial extent of the second vane stages, thepiston defining slots extending into the upstream end of the piston forreceiving the second vane stages.
 6. The turbine assembly of claim 5,wherein the step is in the outer surface of each vane.
 7. The turbineassembly of claim 5, wherein the step is in both the outer and innersurfaces of the vanes.
 8. The turbine assembly of claim 1, wherein theupstream end of the piston defines a radially outwardly extendingflange, the flange defining apertures therethrough for receiving thesecond vane stages, and the flange abutting the step to define theclosed position of the piston.
 9. The turbine assembly of claim 8,wherein the step is in both the outer and inner surfaces of each vane.10. The turbine assembly of claim 1, wherein the upstream end of thepiston has a radial wall thickness exceeding a radial extent of thesecond vane stages, the piston defining slots extending into theupstream end of the piston for receiving the second vane stages, andwherein the step is in both the outer and inner surfaces of the vanes.11. The turbine assembly of claim 1, wherein the generally annular wallto which the vanes are affixed comprises a heat shield.
 12. A turbineassembly for a turbocharger, comprising: a turbine housing defining abore and defining a chamber surrounding the bore for receiving exhaustgas to be directed into a turbine wheel disposed in the bore, a radiallyinner side of the chamber having an axial length; and a variable nozzlehaving stepped two-stage vanes, the variable nozzle comprising: atubular piston disposed in the bore of the turbine housing such that thepiston is axially slidable relative to the turbine housing along theradially inner side of the chamber such that the piston blocks avariable portion of the axial length of the chamber depending on axialposition of the piston, the piston having an upstream end and adownstream end with respect to a flow direction of exhaust gas along thebore of the turbine housing; and a generally annular wall that extendsgenerally radially inwardly adjacent an upstream end of the axial lengthof the chamber, and an array of circumferentially spaced vanes havingfixed ends joined to the generally annular wall and opposite free ends,the vanes extending across the axial length of the chamber, the vaneseach having an outer surface that faces generally radially outwardly andan opposite inner surface that faces generally radially inwardly,wherein each vane defines a first vane stage proximate the fixed end ofthe vane and a second vane stage proximate the free end of the vane,each vane comprising a step transitioning from the first vane stage tothe second vane stage, and wherein the piston in a closed positioncloses the second vane stage so that exhaust gas flows only through thefirst vane stage, the second vane stage being progressively opened asthe piston is axially slid toward an open position, wherein the step isin the outer surface of each vane, and the upstream end of the pistonhas a radially outer surface in which recesses are formed forrespectively receiving the second vane stages with the inner surface ofeach second vane stage confronting a radially outwardly facing wall ofeach respective recess, and wherein the free ends of the vanes abut endwalls of the recesses to define the closed position of the piston.
 13. Aturbine assembly for a turbocharger, comprising: a turbine housingdefining a bore and defining a chamber surrounding the bore forreceiving exhaust gas to be directed into a turbine wheel disposed inthe bore, a radially inner side of the chamber having an axial length;and a variable nozzle having stepped two-stage vanes, the variablenozzle comprising: a tubular piston disposed in the bore of the turbinehousing such that the piston is axially slidable relative to the turbinehousing along the radially inner side of the chamber such that thepiston blocks a variable portion of the axial length of the chamberdepending on axial position of the piston, the piston having an upstreamend and a downstream end with respect to a flow direction of exhaust gasalong the bore of the turbine housing; and a generally annular wall thatextends generally radially inwardly adjacent an upstream end of theaxial length of the chamber, and an array of circumferentially spacedvanes having fixed ends joined to the generally annular wall andopposite free ends, the vanes extending across the axial length of thechamber, the vanes each having an outer surface that faces generallyradially outwardly and an opposite inner surface that faces generallyradially inwardly, wherein each vane defines a first vane stageproximate the fixed end of the vane and a second vane stage proximatethe free end of the vane, each vane comprising a step transitioning fromthe first vane stage to the second vane stage, and wherein the piston ina closed position closes the second vane stage so that exhaust gas flowsonly through the first vane stage, the second vane stage beingprogressively opened as the piston is axially slid toward an openposition, further comprising a tubular carrier inserted into the bore ofthe turbine housing, the piston being received within the carrier andbeing axially slidable relative to the carrier, wherein the carrier isaxially split such that the carrier is able to radially expand andcontract as the piston expands and contracts.